There is now a large body of evidence showing that tumor recruitment of immune cells, particularly tumor-associated macrophages (TAMs), is often correlated with malignant progression and poor prognosis. TAMs originate from the circulating monocytes and, whence in the tumor microenvironment, produce tumorpromoting factors and immune modulators to stimulate tumor growth, invasion, angiogenesis, and metastasis in most, if not all, cancers, including head and neck squamous cell carcinoma (HNSCC). However, the molecular mechanisms that modulate TAM trafficking and subsequent cancer progression are largely unknown. We have found that tumor cells in oral carcinoma in situ (CIS) overexpress human &#946;-defensin-3 (hBD-3), an antimicrobial peptide of the innate immune system. HBD-3 overexpression is correlated with accumulation of macrophages in oral CIS lesions, in xenograft tumors overexpressing hBD-3 in nude mice and in Matrigel plugs supplemented with hBD-3 in immunocompetent mice, without involvement of monocyte chemoattractant protein-1 (MCP-1/CCL2). In addition, hBD-3 increases the incidence and rates of growth of tumors as well as angiogenesis in nude mice. HBD-3 chemoattracts monocytes via the chemokine receptor CCR2 and stimulates tumor-promoting cytokine expression in macrophages, thereby inducing oral cancer cell proliferation and migration. Moreover, our preliminary data show that tumor cell-produced hBD-3 is associated with recruitment of myeloid-derived suppressor cells (MDSCs) in oral CIS lesions and in tumors overexpressing hBD-3 in nude mice. These findings suggest a novel role of the innate immune system in oral carcinogenesis. Therefore, we hypothesize that oral tumor cell-produced hBD-3 recruits and activates TAMs, which in turn suppress antitumor immunity and promote tumor development and progression. To test this hypothesis, we propose in vitro and in vivo assays with the following specific aims: (1) determine the role of hBD-3 in promoting tumor progression in vitro and in vivo. Signaling pathways of tumor cell proliferation and migration in response to hBD-3-activated macrophages will be determined by pharmacological inhibitors and transcription factor profiling analysis. Neutralizing antibodies will then be used to identify proteins secreted by hBD-3-activated macrophages with tumor-promoting activity. An orthotopic mouse model will be generated using mouse oral cancer cells with or without hBD-3 overexpression for tumorigenesis studies. (2) determine the role of TAMs in tumorigenicity associated with hBD-3 in vivo. The effect of hBD-3 on localization and polarization of TAMs as well as tumorigenicity will be examined in vivo. The interplay of hBD-3 with TAMs, MDSCs as well as tumorgenesis will be investigated using a macrophage depletion approach. (3) investigate the molecular mechanism of myeloid cell function in tumors. The role of CCR2 in TAM recruitment by hBD-3 will be examined in vivo using the Ccr2-null mouse model. Migration, expansion and retention of MDSCs in response to hBD-3 will be investigated using in vitro and in vivo assays.